Add overlays output to .gitignore

Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>

Author: bemug-st

.gitignore

@@ -14,4 +14,6 @@
 *.symtypes
 *.order
 modules.builtin
-Module.symvers
+Module.symvers
+*.dtb
+*.dtbo